This invention relates to the recovery of oil by waterflooding from an oil-bearing subsurface formation where said subsurface formation also contains water-sensitive clays.
Oil may be recovered from an oil-bearing subsurface formation by the well-known process of waterflooding. This process basically involves the injection of water into the formation by means of one or more injection wells to physically displace or push oil to production wells. Various improvements have been made in the waterflooding art and have become standard practice. For example, it is well-known to add surfactants to flood water to reduce the interfacial tension between the floodwater and the oil which is to be displaced. It is also well-known that the mobility of the driving fluid, i.e., the floodwater, should be lower than the mobility of the driven fluid, i.e., oil, at least at the interface where these two fluids meet. In other words, a mobility ratio, defined as the mobility of the driving fluid divided by the mobility of the driven fluid, of less than unity is preferred since this prevents the driving fluid from fingering through or bypassing the driven fluid.
The above and other features of the waterflooding art are discussed in more detail in U.S. Pat. No. 3,407,956 issued to Boston, et al., on Oct. 7, 1969, and U.S. Pat. No. 3,677,344 issued to Hayes, et al., on July 18, 1972. These two patents are hereby incorporated by reference. In both of these patents it is recognized that mobility control can be exercised by modification of formation permeability in addition to the more well-known method of modification of fluid viscosity. Mobility is simply the permeability of a formation to a fluid divided by the fluid's viscosity. Mobility may be reduced by either increasing viscosity or decreasing permeability, or doing both. In both of these patents, permeability of the formation to the driving fluid is reduced by interaction with clays present in the more permeable portions of the formation. As defined in these patents, and as used here the term, "water-sensitive clays," includes any clay which swells and/or disperses or in any other way interacts with fresh water to reduce the formation permeability.
In the Hayes patent the mobility of a plurality of banks of flood fluids is successively reduced by reducing the salinity of each bank of fluid. In addition, some of the fluids have polymer thickeners added to increase fluid viscosity. While the Hayes patent involves a process beginning with injection of a micellar fluid to remove oil, the Boston patent deals with a more conventional waterflood having a surfactant added to reduce interfacial tension between the floodwater and the oil being driven through the formation. Disclosed in the Boston patent is the fact that a surfactant solution, including preferably a hydrocarbon sulfonate, having a salinity less than the normal formation salinity, interacts with water-sensitive clays in the formation to reduce formation permeability even when the salinity of the water is sufficiently high that the water alone will not be expected to cause such clay interactions. Thus, it is seen that in both Hayes and in Boston it is taught that a reduction of floodwater salinity below normal formation salinity is essential to the process of reducing formation permeability by in the interactions with clays naturally present in the formation.